FIG. 3 is a circuit diagram of a line terminal device 501 including the conventional signal separation circuit 51.
In the line terminal device 501, the signal separation circuit 51 separates direct electric current power from line signals supplied from the telephone line through a bridge diode BD and a hook switch HS, and dissipating the direct-current power in a load resistance RW.
In other words, in the signal separation circuit 51, the base current for a DC separation transistor Q1 is supplied using the direct-current component of the line signal by a base resistor RB and a base capacitor CB, thereby separating the direct-current power between the collector and the emitter.
Since the base resistance RB is fixed in the conventional signal separation circuit 51 described above, the voltage between the collector and the emitter fluctuates depending on the changes in line current I to produce fluctuation of the collector loss.
Thus, the problem encountered in the conventional signal separation circuits is that, the DC separation transistor Q1 has to be selected by estimating the maximum collector loss, and transistor having a large rating has to be used.